CBM 1541 drive faults

Mike van Bokhoven mike at fenz.net
Sat Dec 12 20:27:08 CST 2015

On 13/12/2015 2:01 p.m., Jules Richardson wrote:
> I have four 1541's now, two of which seem to init OK, and two of which 
> sit there with the drive spindle constantly spinning and the access 
> LEDs lit, the latter behavior which I believe indicates a multitude of 
> possible microcontroller-related faults...
> I've done the obvious, reseating socketed ICs, checking the +12V and 
> +5 rails, and checking the on-board CPU reset line. Does anyone have 
> any tips for what's best to try next?

Locked on LEDs and spindle motor would suggest VIA to me, though it's 
been many years since I've looked at one. First thing I'd check is that 
there's activity on the address/data buses. If things are going on, and 
there are 6522s at hand, I'd try a quick swap.

> I read somewhere that ROM faults aren't unheard of, ditto with 6522 
> VIA failures. Also the 74ls14 at UA1 (particularly if someone's 
> unplugged the drive from the host with power on), but I think that 
> affects CPU reset, which appears to be working in my units.

Can you swap ROMs in from one of the booting drives? Worth a go, though 
I've never seen a ROM failure.

> (is the firmware interchangeable between different board revisions? 
> i.e. can I swap in firmware from one of the drives that appear to init 
> OK to rule that out as a fault, or do I have to worry about matching 
> PCB revisions?)

I wouldn't have thought it'd matter, though maybe the 1541 II was 
different enough.

> Also, has there been a worse external drive in the history of floppy 
> storage? Slow, complicated, expensive and unreliable seems to have 
> most bases covered ;-)

I sort of agree, though I have to say that of my pile of about 30 1541s 
and 1541 IIs, all but two ran up fine on first test, after having been 
in storage for 25 years. One had a bad 6522, and the other had an 
alignment issue. They were all still amazingly slow though!


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